store: refactor access hash and session tables

* Move sessions to user_login metadata, as that data rarely changes after login.
* Merge user and channel access hashes. Those IDs don't conflict.
* Split usernames into a new table to allow better `ON CONFLICT` updates
  (when a username moves to another entity, we want the old row to be replaced).
  Usernames also don't need to be scoped to a login.

pkg/connector/matrix.go

@@ -457,13 +457,9 @@ func (t *TelegramClient) HandleMatrixReadReceipt(ctx context.Context, msg *bridg
 			})
 		case ids.PeerTypeChannel:
 			var accessHash int64
-			var found bool
-			accessHash, found, readMessagesErr = t.ScopedStore.GetChannelAccessHash(ctx, t.telegramUserID, id)
+			accessHash, readMessagesErr = t.ScopedStore.GetAccessHash(ctx, id)
 			if readMessagesErr != nil {
 				return
-			} else if !found {
-				readMessagesErr = fmt.Errorf("channel access hash not found for %d", id)
-				return
 			}
 			_, readMessagesErr = t.client.API().ChannelsReadHistory(ctx, &tg.ChannelsReadHistoryRequest{
 				Channel: &tg.InputChannel{ChannelID: id, AccessHash: accessHash},